# Psychrometric Chart for Design ## Table of Contents - [[#Overview]] - [[#What the Psychrometric Chart Represents]] - [[#Reading the Chart Axes and Lines]] - [[#Dry-Bulb Temperature]] - [[#Wet-Bulb Temperature]] - [[#Dew Point Temperature]] - [[#Relative Humidity]] - [[#Humidity Ratio]] - [[#Enthalpy]] - [[#Specific Volume]] - [[#Identifying a State Point]] - [[#The Comfort Zone on the Psychrometric Chart]] - [[#Givoni Bioclimatic Overlay]] - [[#Passive Strategy Zones]] - [[#Plotting Climate Data]] - [[#Practical Applications for Architects]] - [[#Key References]] --- ## Overview The psychrometric chart is one of the most powerful graphical tools available to the environmental designer. It maps the thermodynamic properties of moist air onto a single diagram, enabling the architect to visualise climate conditions, identify comfort gaps, and select appropriate passive and active strategies. When combined with Givoni's bioclimatic overlay, the chart becomes the primary instrument for climate-responsive design decision-making. This article explains how to read and use the chart for architectural design purposes. It connects directly to [[Thermal Comfort Fundamentals]] for comfort criteria, [[Bioclimatic Architecture]] for the broader design methodology, and [[Passive Solar Design]] for one of the key strategies identified on the chart. --- ## What the Psychrometric Chart Represents The psychrometric chart is a graphical representation of the thermodynamic properties of air-water vapour mixtures at a constant atmospheric pressure (typically standard pressure of 101.325 kPa at sea level). Every point on the chart represents a unique state of moist air defined by any two independent properties. The chart is constructed at a specific altitude. For high-altitude locations (above approximately 900 m), altitude-corrected charts should be used, as reduced atmospheric pressure shifts the property lines. --- ## Reading the Chart Axes and Lines The standard ASHRAE-format psychrometric chart uses the following layout: - **Horizontal axis (x-axis):** Dry-bulb temperature - **Vertical axis (right side):** Humidity ratio (moisture content) - **Curved upper boundary:** Saturation line (100% relative humidity) - **Diagonal lines sloping left:** Wet-bulb temperature and enthalpy - **Curved lines within the chart:** Relative humidity contours - **Near-vertical lines from the horizontal axis:** Specific volume (sometimes omitted in simplified charts) --- ## Dry-Bulb Temperature **Definition:** The temperature of air measured by a standard thermometer, unaffected by moisture content. **Symbol:** T_db (degC) **On the chart:** Read along the horizontal axis at the bottom. Vertical lines of constant dry-bulb temperature extend upward. **Design relevance:** Dry-bulb temperature is the most commonly reported climate parameter and directly affects sensible heating and cooling loads. --- ## Wet-Bulb Temperature **Definition:** The temperature measured by a thermometer with its bulb wrapped in a wet wick exposed to air flow. It represents the lowest temperature achievable by evaporative cooling of the air. **Symbol:** T_wb (degC) **On the chart:** Diagonal lines sloping from upper left to lower right. The wet-bulb temperature at the saturation curve equals the dry-bulb temperature at that point. **Design relevance:** The **wet-bulb depression** (T_db - T_wb) indicates the potential for evaporative cooling. A large depression (dry climate) means evaporative cooling is highly effective. A small depression (humid climate) means evaporative cooling offers little benefit. ### Example At T_db = 40 degC and RH = 20%: T_wb approximately equals 23 degC Wet-bulb depression = 40 - 23 = 17 degC -- excellent evaporative cooling potential At T_db = 32 degC and RH = 80%: T_wb approximately equals 29 degC Wet-bulb depression = 32 - 29 = 3 degC -- negligible evaporative cooling potential --- ## Dew Point Temperature **Definition:** The temperature at which air becomes saturated (100% RH) if cooled at constant moisture content. Below this temperature, condensation occurs. **Symbol:** T_dp (degC) **On the chart:** From any state point, move horizontally left to the saturation curve. The temperature at that intersection is the dew point. **Design relevance:** - Condensation risk assessment: if any surface temperature drops below T_dp, condensation will form - Critical for specifying insulation thickness, cold bridge avoidance, and glazing selection - Dehumidification in HVAC systems cools air below T_dp to remove moisture --- ## Relative Humidity **Definition:** The ratio of the actual water vapour pressure to the saturation vapour pressure at the same temperature, expressed as a percentage. **Symbol:** RH (%) **On the chart:** Curved lines running roughly parallel to the saturation curve, labelled 10%, 20%, 30%, etc. The saturation curve itself is 100% RH. **Design relevance:** - Comfort range: generally 30-60% RH for indoor environments - Below 30% RH: dry skin, static electricity, respiratory irritation - Above 60% RH: mould risk, dust mite proliferation, perceived stuffiness - Above 80% RH: condensation risk on cool surfaces, material degradation --- ## Humidity Ratio **Definition:** The mass of water vapour per unit mass of dry air. **Symbol:** W (g/kg or kg/kg) **On the chart:** Read on the right-hand vertical axis (or sometimes the left). Horizontal lines of constant humidity ratio extend across the chart. **Design relevance:** Humidity ratio is constant during sensible heating or cooling (no moisture added or removed). It is the key parameter for sizing dehumidification and humidification systems. --- ## Enthalpy **Definition:** The total energy content of moist air per unit mass of dry air, including both sensible and latent components. **Symbol:** h (kJ/kg) **On the chart:** Diagonal lines, often on the same scale as wet-bulb temperature, extending beyond the saturation curve to a peripheral scale. **Design relevance:** Enthalpy determines the total cooling or heating load, including both temperature change (sensible) and moisture change (latent). Two air states with the same dry-bulb temperature but different humidity ratios will have different enthalpies. --- ## Specific Volume **Definition:** The volume occupied by one kilogram of dry air plus its associated moisture. **Symbol:** v (m3/kg) **On the chart:** Near-vertical lines, slightly inclined, typically ranging from 0.78 to 0.92 m3/kg in the normal comfort range. **Design relevance:** Used to convert between volumetric flow rate (m3/s) and mass flow rate (kg/s) in ductwork and ventilation calculations. --- ## Identifying a State Point To plot any air condition on the chart, two independent properties are needed. Common combinations: | Known Properties | Procedure | |-----------------|-----------| | T_db + RH | Find T_db on x-axis, follow vertical line up to the RH curve; intersection is the state point | | T_db + T_wb | Find T_db on x-axis and T_wb on the diagonal; intersection is the state point | | T_db + T_dp | Find T_dp on the saturation curve, draw horizontal line to T_db vertical; intersection is the state point | | T_db + W | Find T_db on x-axis, W on y-axis; intersection is the state point | From the state point, all other properties (those not used to define it) can be read directly. --- ## The Comfort Zone on the Psychrometric Chart The ASHRAE 55 comfort zone is plotted on the psychrometric chart as a bounded region: ### Summer (0.5 clo, 1.0-1.2 met) - Operative temperature: approximately 23.5-27.0 degC - Humidity ratio upper limit: approximately 12 g/kg (equivalent to approximately 17 degC dew point) - No explicit lower humidity limit in ASHRAE 55 ### Winter (1.0 clo, 1.0-1.2 met) - Operative temperature: approximately 19.5-24.0 degC - Same humidity limits The comfort zone shifts left (cooler) in winter due to heavier clothing insulation. It can be extended upward and to the right with elevated air speed (see [[Thermal Comfort Fundamentals]]). --- ## Givoni Bioclimatic Overlay Baruch Givoni's building bioclimatic chart is a set of strategy-zone boundaries overlaid on the psychrometric chart. Each zone identifies climatic conditions where a specific passive strategy can restore comfort. ### Strategy Zones (Approximate Boundaries) | Zone | Description | Approximate Bounds | |------|------------|-------------------| | **Comfort** | No intervention needed | 20-27 degC, 20-80% RH (varies with air speed) | | **Natural ventilation** | Air movement restores comfort | 27-32 degC at low-moderate humidity | | **Evaporative cooling** | Direct or indirect evaporative cooling | Hot and dry conditions (RH < 40%, T_db up to 44 degC) | | **Thermal mass** | High mass absorbs daytime heat | Hot and dry, moderate humidity, diurnal range > 10 degC | | **Thermal mass + night ventilation** | Mass cooled by night air purge | As above but with greater cooling demand | | **Passive solar heating** | Direct/indirect solar gain | Cool conditions below comfort zone | | **Conventional heating** | Active heating required | Cold, below passive solar potential | | **Conventional cooling** | Active cooling required | Hot and humid, beyond passive reach | ### How to Use the Overlay 1. Obtain hourly climate data for the site (TMY file) 2. Plot each hour as a point on the psychrometric chart (or use software such as Climate Consultant) 3. Overlay Givoni strategy boundaries 4. Count the number of hours falling within each zone 5. Rank strategies by the number of discomfort hours they address 6. The strategy addressing the most hours should dominate the design approach --- ## Passive Strategy Zones ### Natural Ventilation Zone Extends the comfort zone to higher temperatures (up to approximately 32 degC) where air movement at 1.0-1.5 m/s can provide adequate cooling effect. Relevant in hot humid climates where evaporative cooling is ineffective. ### Evaporative Cooling Zone Extends to the right of the comfort zone into high-temperature, low-humidity conditions. The wet-bulb temperature in this zone is below the comfort range, meaning evaporative processes can cool the air sufficiently. Most applicable in hot arid climates (see [[Bioclimatic Architecture]]). ### High Thermal Mass Zone Located above the comfort zone in hot, dry conditions. Buildings with high thermal mass (time lag 8-12 hours) can absorb daytime heat gains and release them during cooler night hours. Combined with night ventilation, this zone extends further. ### Passive Solar Heating Zone Below the comfort zone, where solar gains through south-facing glazing (see [[Passive Solar Design]]) can raise indoor temperatures to comfortable levels. The extent of this zone depends on insulation quality, glazing area, and available solar radiation. ### Humidification / Dehumidification Conditions too dry or too humid for comfort may require moisture addition or removal. These zones appear at the left (dry) and upper right (humid) of the chart. --- ## Plotting Climate Data ### Manual Method 1. Obtain monthly average or hourly T_db and RH for the site 2. Plot each data point on the chart using the method described in [[#Identifying a State Point]] 3. Connect monthly points to visualise the annual climate trajectory 4. Identify which months fall outside the comfort zone and in which direction ### Software Method | Tool | Output | |------|--------| | Climate Consultant (UCLA) | Psychrometric chart with Givoni overlay, hourly data, strategy percentages | | Ladybug (Grasshopper) | Psychrometric chart with customisable comfort models | | Weather Tool (Autodesk) | Psychrometric chart with basic strategy zones | | CBE Thermal Comfort Tool | Web-based psychrometric plotter with ASHRAE 55 zones | Climate Consultant is the most accessible free tool for this analysis and is recommended for preliminary design. --- ## Practical Applications for Architects ### Pre-Design Climate Analysis Before any design decision, plot the site climate on the psychrometric chart with Givoni overlay. This immediately reveals: - Whether the climate is predominantly heating or cooling-dominated - Which passive strategies have the greatest potential - How many hours can be addressed passively versus requiring mechanical systems - Whether humidity is a primary challenge ### Strategy Selection Example **Site: Cairo, Egypt** - Winter: data points fall in the passive solar and comfort zones - Summer: data points fall in the evaporative cooling, thermal mass, and natural ventilation zones - Conclusion: high thermal mass, evaporative cooling, and passive solar heating are the primary strategies **Site: Singapore** - Year-round: data points cluster in the natural ventilation zone and conventional cooling zone - Evaporative cooling zone is barely used (already humid) - Conclusion: maximise natural ventilation; mechanical cooling needed for peak conditions ### Design Verification After establishing the building design, re-plot predicted indoor conditions on the psychrometric chart to verify that comfort is achieved. Dynamic simulation outputs can be plotted hourly to assess annual performance. ### Communication Tool The psychrometric chart is an effective communication device for explaining climate strategy to clients and design teams. A single chart can demonstrate why specific design decisions (orientation, mass, ventilation, shading) are appropriate for the site. --- ## Key References - ASHRAE. *Handbook of Fundamentals* -- Psychrometrics chapter - Givoni, B. (1998). *Climate Considerations in Building and Urban Design* - Lechner, N. (2014). *Heating, Cooling, Lighting* - Szokolay, S. (2008). *Introduction to Architectural Science: The Basis of Sustainable Design* - Milne, M. and Givoni, B. (1979). "Architectural Design Based on Climate" in *Energy Conservation Through Building Design* - UCLA Energy Design Tools Group. *Climate Consultant* software (free download) --- #environment #psychrometric